Weather resistant granules of various hues, embedded on the surface of various types of roofing and/or siding, have been used extensively to provide an aesthetically pleasing color to such surfaces. In North America, asphalt-based roofing materials are a popular medium for covering roofs of homes and other structures. Asphalt-based roofing materials typically come in shingle or roll form, the shingle being the more widely used material. A typical asphalt shingle has an asphalt substrate and a multitude of granules placed thereon. The granules are generally embedded in the asphalt coating on the surface of an asphalt-impregnated base material such as roofing felt. The granules, which are embedded in one surface of the asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and/or decorative exterior surface. Because the granule coating provides the aesthetic effect observable with respect to the roofing material, the appearance of the granules is of major marketing importance. For this reason, a pigmented color coating is ordinarily applied to the granules to enhance their visual decorative effect.
The granules employed for roofing and siding are generally derived from a hard mineral base rock such as slate, basalt or nephelite. The granules are typically ground to a particle size of about 10 to about 35 US mesh. These granules, which comprise a base substrate, are then coated with a pigment composition having a thickness of about 4 to about 18 microns. Colored roofing granules are typically prepared by heating mineral rock granules of about 12-40 US mesh up to approximately 1000° F. A paint slurry containing a pigment is then applied to the heated granules in a mixer. Kaolin clay is used extensively in silicate paint formulations for coloring roofing granules. It serves as a filler, extender, moisture release agent, and reactant to aid film insolubilization during high temperature firing. Although kaolin clay is a major component of such coating formulations, it alone lacks the brightness and opacity needed to hide the dark underlying base rock of the granule. White colored roofing granule coatings using natural kaolin clay frequently require appreciable amounts of TiO2 to achieve desired color specifications. White or light colored roofs are particularly favored in many areas. The TiO2 is commonly used in conventional insolubilized alkali silicate coatings, such as those described in U.S. Pat. Nos. 2,379,358; 3,255,031; and 3,479,201, which are incorporated herein by reference. Other types of coating compositions have been used. For example, U.S. Pat. No. 3,528,842, which is incorporated herein by reference, discloses artificially colored roofing granules. These granules consist primarily of crushed and screened minerals, in which the granules are coated with a suitable pigment contained in an inorganic matrix or bond. Some common pigments include red iron oxide, yellow iron oxide, titanium dioxide, chrome hydrate, chrome oxide, chrome green, ultramarine blue, phthalocyanine blue and green, carbon black, metal ferrites, and mixtures thereof. The bond is usually applied in the form of a soluble silicate solution and is insolubilized either by heat treatment or by a combination of heat treatment and chemical action. Minnesota Mining and Manufacturing Company (“3M”) has offered pigmented roofing granules known under the trade designations “LR-7000”, “LR-7070”, 3M Brand “4100”, and 3M Brand “9300”. These granules include multiple silicate-clay coatings which may include cuprous oxide or zinc oxide. Other types of pigment compositions include an aqueous slurry of an iron oxide pigment and a mixture of a silicate and clay.
Although the color of a particular roofing system plays an important role in determining which roofing system will be purchased by consumers, other factors are also gaining more prominence in determining the color of a particular roofing system. One such factor is the energy efficiency of the roofing system. This is of particular importance in more temperate regions, such as in the Southern and Southwest regions of the United States. Lighter colored roofs are known to reduce the roof temperature, thereby reducing the cooling costs of the roofed structure. For instance, on a 90° F. sunny day, the roof temperature of a white granular coated roof will be 20-30° F. cooler than an aluminum coated roof and 70-80° F. cooler than a black asphalt roof. As a result, the lighter colored roof system can amount to an energy savings of about 5-10%. The cooler temperature of the white roof system not only reduces the cooling costs associated with the building, but the heat generated by the white roof contributes less heat to the surrounding environment, thus improving air quality, especially in urban areas.
Typical light colored roofing systems have light colored granules which have a reflectivity of up to 30% and a prepared roofing reflectivity of up to 27%. Darker colored granules typically have a lower reflectivity and a resulting roofing reflectivity than lighter colored granules. In view of the continued demand for greater energy savings, there is a demand for a roofing system having increased reflectivity.